Congress is in the midst of a heated debate over gun control and how best to tamp down on gun violence. And mass shootings like the one that tore through Sandy Hook Elementary School last December are a huge part of that discussion.

So how much do we actually know about mass shootings and the people who commit them? A new study (pdf) commissioned by Mayors Against Illegal Guns* tries to add some much-needed detail.

The researchers pored through the FBI database and recent media reports for every mass shooting since January 2009 — that is, incidents in which at least four people were murdered by guns. Here are their key takeaways, some of them surprising:

1) Mass shootings have occurred at an average rate of about one per month since 2009. The report concludes that there have been 43 mass shootings in 25 states over the past four years — or nearly one per month.

2) Yet mass shootings are still a tiny portion of overall gun deaths. For all the attention they receive, mass shootings are not the main source of gun violence. In 2010, according to the FBI, around 8,775 people were murdered with firearms in the United States. Less than 1 percent of those victims were killed in mass shootings.

3) Assault weapons are used in a minority of mass shootings — but those incidents were much deadlier. Just 12 of the mass-shooting incidents, or 28 percent, involved assault weapons or high-capacity magazines — the very same guns that some members of Congress are now trying to ban. At the same time, mass shootings were a lot deadlier when assault weapons and high-capacity magazines were used, with an average of 8.3 deaths, compared with 5.4 deaths on average for the rest.

4) Few mental-health red flags came up before most of the shootings. In just four of the 43 shootings was there evidence that someone had raised concerns about the mental health of the killer to authorities beforehand. Likewise, the report notes, there was no evidence that any of the shooters had been prohibited from owning firearms because of mental-health concerns.

5) Domestic violence played a role in 40 percent of mass shootings. From the report: "In at least 17 of the cases (40%), the shooter killed a current or former spouse or intimate partner, and at least 6 of those shooters had a prior domestic violence charge."

6) At least 11 of the shooters were prohibited from owning guns. Under federal law, felons, certain domestic abusers and people deemed mentally ill are barred from owning guns. The report found that at least 11 of the shooters fell into this category — although there was no good data one way or the other for about one-third of the cases.

7) About one-third of the shootings took place in gun-free zones. Some additional stats: "Nineteen of the 43 incidents (44%) took place in private residences. Of the 23 incidents in public spaces, at least 9 took place where concealed guns could be lawfully carried. All told, no more than 14 of the shootings (33%) took place in public spaces that were so-called “gun-free zones.”

* It's worth noting that Mayors Against Illegal Guns, founded by New York City Mayor Michael Bloomberg, is a group that strongly supports stricter gun-control laws. But for those who want to double-check their claims, the group lists every single mass shooting incident — as well as the relevant details — in the report.

Further reading:

— The full report (pdf).

— This week, the Department of Homeland Security released its own study looking at the profile of mass shooters since 1999. One key takeaway? They tended to be young males acting alone using handguns. And very few were ex-military.

— The last assault-weapons ban didn’t work. Will the new one be different?